Different types of machines are available for making plastic bags in rolls. These machines generally operate according to the principle that a web consisting of a flattened plastic film tube is fed into the machine in which the web is provided with transverse weld lines and lines of perforation. The weld lines form the seals of the bags, whereas the lines of perforation define the bag lengths. At the end of or after the machine, the web provided with these lines is rolled up on rolls. Most prior art machines of this type have some units in common, viz. a feeding unit, a perforation unit, a welding unit, a cooling unit, a discharge unit and a rolling-up unit.
An example of such a machine is disclosed in U.S. Pat. No. 4,950,217 and the counterpart EP-A-0,333,726. This prior art machine comprises a welding unit having two opposite chain and guide pulley devices with transverse sealing jaws which are pressed against each other so as to clamp the web descending through the machine. This machine functions in a per se satisfactory manner, but has certain limitations as to the speed of the web, which depends on, inter alia, the relatively short time available for the welding operation as the web is fed through the welding unit. Normally, this and similar machines can be run at a maximum web speed of about 260-295 feet/min (80-90 m/min). However, there are increasing demands on the market that bag-making machines should be operable at considerably higher web speeds, preferably about 500 feet/min (150 m/min), which is one of the basic points of the present invention.
One reason why a bag-making machine that can stand up to high speeds of the web should be developed is that modern extruders for producing the plastic film tube to be fed into the bag-making machine are operated at speeds of at least 400 feet/min (120 m/min). Since such an extruder is usually positioned directly before a bag-making machine, it must of course be aimed at the two machines being capable of operating at substantially the same speed.
Other types of bag-making machines are provided with a rotary drum, the web being passed over part of the circumferential surface of the drum so as to be welded by means of sealing jaws disposed thereon. U.S. Pat. No. 4,380,446 discloses such a machine having a vertical drum with a number of sealing jaws which extend along generatrices on the circumferential surface of the drum. As seen from the end of the drum, the sealing jaws are distributed equiangularly with respect to the shaft of the drum. The radial distance between the sealing jaws and the shaft of the drum is to some extent adjustable for providing different spacings between the weld lines. This "spoke arrangement" with a substantially fixed angular distribution of the sealing jaws yields, however, limitations both as to the available range of bag length and as to the maximum web speed.
U.S. Pat. No. 4,567,984 and 4,642,084 disclose a bag-making machine which is also provided with a sealing or welding drum which in this case is horizontal and rotated by a belt laid around part of the circumferential surface of the drum, the web being passed therebetween. Over the circumferential surface of the drum a number of sealing jaws are arranged, which extend along generatrices of the drum and transversely of the direction of travel of the web. The sealing jaws are supported at each end of the drum by a spoke arrangement connected to the drum shaft. For different bag lengths, the diameter of the drum is variable in that the radial distance between the sealing jaws and the shaft of the drum is adjustable. Thus, short bags are run at a small drum diameter, and long bags at a large drum diameter. To this end, the spokes included in the spoke arrangement, which at their free end carry the sealing jaws, are hinged bars which are fixedly mounted on a hub of the drum shaft and distributed equiangularly.
However, the welding unit of this known bag-making machine has certain limitations and drawbacks as will be described below.
The available bag length interval is restricted and depends on the variation in diameter of the drum. The lower limit of the bag length is determined by the angular distribution of the spokes carrying the sealing jaws, whereas the upper limit is determined by the maximum diameter of the drum. To date, the possible range of bag length has been about 6-70 inches (150-1800 mm).
The welding distance, i.e. the circular arc along which a sealing jaw is engaged with the web for welding, varies with the bag length. When producing short bags, the welding distance is short, which may jeopardise the quality of the welding. This problem is, of course, further accentuated if at the same time the rate of production is high, since the welding time will then be too short.
The sealing jaws cannot be controlled individually, since they are connected directly to the rotation of the drum, which limits the flexibility of the machine.
The spoke arrangements which support the sealing jaws and enable the variation in drum diameter are rather complicated.
The relevant background art also includes the bag-making machine according to U.S. Pat. No. 4,934,993, which is a further development of the machine disclosed in the two U.S. patents discussed above. According to this last variant, the diameter of the welding drum can be set during operation by means of a special device which actuates the spoke arrangements positioned at each end of the drum and carrying the sealing jaws. Nevertheless, this prior art machine suffers from the same drawbacks as its predecessor.